pubsub: improve Client.Close doc

Clarify that Close releases goroutines and memory, and try
to explain better when you don't have to call it.

Updates #711.

Change-Id: I7f2fca57da4f5bbcd0d3d5a327d365457973c113
Reviewed-on: https://code-review.googlesource.com/20250
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Michael Darakananda <pongad@google.com>

pubsub/pubsub.go

@@ -88,9 +88,11 @@ func NewClient(ctx context.Context, projectID string, opts ...option.ClientOptio
 	return c, nil
 }
 
-// Close closes any resources held by the client.
+// Close releases any resources held by the client,
+// such as memory and goroutines.
 //
-// Close need not be called at program exit.
+// If the client is available for the lifetime of the program, then Close need not be
+// called at exit.
 func (c *Client) Close() error {
 	return c.s.close()
 }